A steel sheet for enamel is used in home appliances, chemical equipment, cooking tools, sanitary appliances, interior and exterior materials for building, and the like.
There is a hot rolled steel sheet or a cold rolled steel sheet as the steel sheet for enamel, but the cold rolled steel sheet is mainly used for the purpose of high function and processing. Examples of the steel sheet for enamel include rimmed steel, OCA steel (open coil aluminum steel), titanium-added steel, high oxygen steel, and the like. Examples of important defects in the steel sheet for enamel include fish scales.
The fish scales mean defects formed when hydrogen gas collected in the steel is discharged between a surface of the steel and an enamel layer to lift a surface of the enamel layer in a form of scales of fish. The fish scales are formed when hydrogen solid-soluted in the steel during a process of manufacturing the steel sheet for enamel is discharged to the surface of the steel in a cooling state but not discharged to the outside because the enamel layer on the surface of the steel is previously hardened.
As described above, since the fish scale defect is caused by hydrogen, it is necessary to form a position where hydrogen can be adsorbed in the steel in order to prevent the defects from being formed.
The adsorption position of hydrogen may be micro-voids, inclusions, deposits, a dislocation, a grain boundary, or the like.
In the case of the rimmed steel, since an oxygen content is high, the inclusion may be generated in a large amount to prevent fish scale defects from being formed. However, since the rimmed steel can be manufactured by only a steel ingot casting method, productivity is not high. Accordingly, an enamel molten steel that can be manufactured by continuous casting having high productivity is required.
A Ti or Nb-added enamel molten steel is manufactured by using a continuous annealing process in order to reduce manufacturing cost. However, since the enamel molten steel has a high re-crystallization temperature, annealing treatment should be performed at high temperatures, and thus there are drawbacks in that productivity is low and manufacturing cost is high.
Further, in the case where the Ti-added steel is continuously cast by added Ti, a nozzle is clogged, and in the case where a large amount of inclusion is exposed to the surface of the steel sheet, bubble defects are generated after an enamel treatment. Further, in the case of the Ti-added steel, added Ti generates inclusions such as TiN, and there is a problem in that the TiN inclusion exists on the surface of the steel sheet to reduce a close contacting property of the enamel.
In addition, the high oxygen steel having the increased oxygen content can ensure a hydrogen occlusion ability by using oxides in the steel.
However, since the high oxygen steel has the high oxygen content in the steel, a fireproof material is melted during continuous casting, and thus productivity by continuous casting is very low.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.